Half-duplex (HDX) transponders are used in “passive” RFID tags that have no autonomous power supply. Instead, they use the RF energy received with an antenna and an LC oscillating circuit from a transmitter during an interrogation interval by rectifying the received RF signal and charging a storage capacitor with the rectified signal. The end of the interrogation interval is detected by an end-of-burst (EOB) detector. The interrogation interval is followed by a response interval where the transponder is expected to send some response, e.g. an ID code or some other data. Energy for operating the transponder's transmitter during the response interval is supplied by the storage capacitor. Since the same antenna and LC oscillating circuit are used in both of the interrogation and response intervals, and the oscillation amplitude of the LC oscillation circuit decreases with time as soon as an RF signal is no longer received from the interrogation transmitter, there a need to maintain oscillation by stimulating the LC oscillation circuit. Energy for such stimulation is available from the storage capacitor.
In a conventional solution (U.S. Pat. No. 6,806,738, EP 1 233 371 Bi), sophisticated circuitry is provided for detecting the negative peak value of the oscillation signal and injecting energy from a low impedance source into the LC oscillating circuit just for a short time after the detection of the negative peak value. By analogy to an acoustic oscillating system, this is referred to as “plucking”.
The “plucking” approach has proven to be efficient, but it is complicated and needs a lot of analog circuitry that is difficult to implement in CMOS technology. Also, the oscillation signal is distorted by the sudden injection of energy during a short fraction of the oscillation period, and a frequency shift is observed.